afinepoint

The Bentley manual does not direct troubleshooting of the DME main/fuel pump module as a source for an erratic idle. Tlhe manual states that if the module malfuctions the engine will not start. Yet I have seen suggestions in this forum to replace it for such a problem as it is prone to malfuction and failure. As well as for carrying a spare.

I feel the source of my occassional erratic idle is electrical. It mainly occurs when the engine is hot, as in city traffic idling, and clears once the engine stalls and is restarted.

I had thought of getting a relay anyway to test the installed one then keeping the new one onboard just in case.

AFP

J. Brinkley

you could just unplug the old one and plug in a new one to see if anything changes, but I would check the temp sensor first. The white colored one on the tree obove the 123 side fuel rail.

afinepoint

I have a new head temp sensor that has been laying around for some time but never installed as I have been chasing other ghosts.

I'll just order the DME and do both next free day.

Thanks,

AFP

ClubsportSP

Also check the DME ground points - think there are 3 in total. FWIW, combination of cleaning ground points and replacing CHT restored a steady idle on my '88 3.2CS

Cheers

Shirish

Ben Fegan

I have an extra complete DME, Part # 911.618.200.20 for an 88 Carrera. Fully operational.

I paid $650 for it, plus a $150 "core" charge (refundable if I sent in my old DME... which I did not).

You can have it for $650 plus your old one and I'll ship for free. A new one is $2,000!

DME works fine.

This is a 28 Pin DME with more memory and would be an upgrade for an 84.

Chris Bennet

I doubt your DME has a problem. DME relays can and do fail and it is wise to carry a spare. I doubt the DME relay has anything to do with your problem however.

At idle, the DME controlled Idle Stabilizer Valve (ISV) allows a certain amount of air to "detour" around the closed throttle plate. The DME uses the ISV to control the idle - if it idles too fast it reduces the amount of air to the motor, if it idles too slow it increase the amount of air.

As long as the motor "wants" to idle close to the where the computer controlled idle is RPM set, everything is fine. The motor will "want" to idle at different RPM's based on various parameters like altitude, how warm the motor is, what its sensors are telling it, etc.

If the parameters that determine the "natural" idle speed change to the point where the DME cannot correct for them, the idle will be erratic.

For example, suppose the gaskets between the intake and the heads shrink and start leaking allowing extra air into the motor. The extra air would raise the idle. The DME would direct the ISV to cut off air to bring the idle down, (possibly almost stalling the motor), now the DME "sees" that the RPM is too low and directs the ISV to allow more air to increase the RPM. The idle raises too fast, over shoots the target idle speed and then the process repeats. RuumpRuumpRuump goes the idle.

Or suppose the O2 sensor is getting old and "lazy". It indicates that the mixture is too lean, the DME richens the mixture. The O2 sensor should now indicate the new richer mixture but because it is old, it has become slow to react and it's still indicating that the mixture is too lean. The DME richens the mixture some more until the O2 sensor finally reacts and indicates that the mixture is too rich. By this time the natural idle has changed enough that the computer has trouble correcting and we get a swinging idle.

You get the idea. Like ClubSport said, my first suspect would be the cylinder head temp (CHT) sensor. If it is the original single wire style just replace it before it porks you on a rainy night 300 miles from home. If you can get the erratic idle to happen in your driveway, you can bypass the CHT by inserting a 270 or 330 ohm resistor into the harness connector for the CHT (assuming motor is hot). The connector from the CHT is the white one at the top in the bracket holding three connectors on the left side of the motor just below the heater blower motor.

Good luck.
-Chris

rbcsaver

I 2nd Chris' post. Probably **NOT** your DME unit.
regards,
Steve

944ZEN

I have recently renovated a Idle Stabilisor Valve (ISV) from a 91' 964. It was only partly working, but its movement was irratic because of oil sludge deposit and bad lubrication, so t would open and close in bigger unpedictable steps. That means that when the DME sends signal to the ISV to open a specific amount it properly wouldnt (stuck) or it would jump a step. So teh DME would have severe probelms controlling idle speed.

The cure was to disassembel the ISV, cleaning it inside and relubricate.

The probelm is that the intake sucks the oil vapour from cranck ventilation, and over time this oil film can build up deposits in the ISV. The ISV has narrow tolerances around the variable air opening, hta can clog up.